Method and device for increasing the intrinsic viscosity of polyester material by means of solid phase polymerisation

ABSTRACT

The invention relates to a process and a device for increasing the intrinsic viscosity of a polyester material by solid-state polymerization, wherein the polyester material is heat-treated in a heat treatment container ( 6 ), the polyester material being introduced into a preheating container ( 2 ) prior to being conveyed into the heat treatment container ( 6 ), in which preheating container it is heated to a heat treatment temperature of the heat treatment container ( 6 ) or to a temperature above that and is delivered to the heat treatment container ( 6 ) after having reached said temperature. The preheating container ( 2 ) is small in comparison with the heat treatment container ( 6 ) so that the relatively small amount of polyester material contained therein can quickly be heated to the intended temperature, thereby leading to a short residence time in the preheating container. After reaching the intended temperature, the entire content of the preheating container can be delivered all at once to the heat treatment container, whereupon the preheating container can be charged with fresh polyester material. Due to the short residence time, the preheating container is operable well in a discontinuous operation.

The invention relates to a process and a device for increasing theintrinsic viscosity of a polyester material by solid-statepolymerization, wherein the polyester material is heat-treated in a heattreatment container.

When producing high-molecular polyesters such as PET and PEN, forexample, a behaviour of polyester which is unique among syntheticmaterials is utilized, according to which behaviour polycondensation ofthe polyester molecules occurs and hence the viscosity of the polyesteris increased if the polyester remains under high temperatures and vacuumor under inert gas in order to prevent the oxidative degradation. Thispreparation of high-molecular polyesters from a low-molecular polyesterstarting material usually occurs via melt polymerization or solid-statepolymerization or a combination of both processes.

In case of melt polymerization, a polyester melt is processed attemperatures of approx. 270° C. to 300° C. for about 30 minutes to 5hours under a strong vacuum of approx. 1 mbar. This involves thedrawback that, due to the high processing temperatures, the initiallydescribed oxidative degradation process of the polyester will take placewhich leads to yellow colouring and counteracts the polycondensation ofthe polyester. The intrinsic viscosity values achievable by meltpolymerization are approximately in the range of 0.6 IV (=IntrinsicViscosity).

In case of solid-state polymerization, the polyester melt is usuallyextruded through several dies, and subsequently the synthetic strandsthereby formed are cooled in a water bath. After having solidified, thesynthetic strands are granulated, i.e. cut to pellets. Due to the rapidcooling, the polyester is provided in the amorphous state. This isimportant since polyester materials which originally were transparentremain translucent in the amorphous state, whereas, if cooling is slow,the polyester assumes a crystalline state in which a material whichoriginally was transparent changes its colour to white. For furtherprocessing, the polyester granulate must be reheated, whereby thegranulated bodies become agglutinated in the range of thecrystallization temperature (80-120° C.). Therefore, the granulate isfirst supplied to a so-called crystallizer in which it is brought to atemperature above the crystallization temperature under vigorousstirring in order to regain the flowability of the granulated bodies forfurther treatment, which is of great importance for the conveyance anddrying in a container without agitator. Moreover, the granulate, in itscrystalline form, absorbs less moisture, thereby permitting shorterresidence times during drying. The granulate is then fed into asolid-state polymerization container, also referred to as an SSP(solid-state polymerization) reactor or heat treatment container,wherein it is heated to approx. 220 to 250° C. and subsequently is leftunder those conditions for about 1-40 hours until the desired intrinsicviscosity has been reached.

The heating of the polyester granulate in the SSP reactor is carried outaccording to the prior art either by means of an inert gas stream (f.i.nitrogen) as a heat-transfer medium, which stream—which is heatedoutside of the reactor—passes through the reactor and the granulatelocated therein, thereby transferring its heat to the granulate, andsubsequently is sucked off; or by means of heating elements in anevacuated reactor.

Heating by an inert gas stream involves the disadvantage that theemployed technical gases (f.i. nitrogen) are expensive and thereforemust be conducted in a closed circuit, also for reasons of environmentalprotection. Said closed circuit also requires that costly cleaningdevices for the inert gas stream must be provided in order to filter outtoxic substances and impurities taken up from the granulate. Theimplementation of such a reactor therefore only pays off with hugeplants having a throughput in the range of 20 tons of polyestergranulate per day and more.

Heating the granulate in a container under vacuum involves disadvantagesin that the vacuum is an excellent heat insulator and, due to thisproperty, counteracts the heating of the granulate. Therefore, it iseither necessary to provide extremely long residence times of thegranulate in the container or, in case of heating via heating elementsattached to the exterior of the container, to provide one or severalagitators for mixing the granulate in the interior of the container, orto provide technically complex movable heating elements in the interiorof the container, which serve simultaneously as mixing elements. Allthose constructional measures, however, give rise to technical problemssuch as the formation of dead spaces in which the granulate gets stuck,turning round of the granulate stream, non-uniform heating, high energyconsumption etc. and are undesirable also for cost reasons. For theabove-mentioned reasons, a continuous charging of the heating containerwith granulate is rendered extremely difficult.

Therefore, it is an object of the present invention to provide a processand a device of the initially mentioned type, by means of which theabove-described disadvantages of the prior art are alleviated or eveneliminated.

In the process according to the invention, this is achieved in that thepolyester material is introduced into a preheating container prior tobeing conveyed into the heat treatment container, in which preheatingcontainer it is heated to a heat treatment temperature of the heattreatment container or to a temperature above that, preferably toessentially the heat treatment temperature, and is delivered to the heattreatment container after having reached said temperature. Generally,the heat treatment temperature amounts to at least 180° C. Thepreheating container may be small in comparison with the heat treatmentcontainer so that the relatively small amount of polyester materialcontained therein can quickly be heated to the intended temperature,thereby leading to a short residence time in the preheating container.After reaching the intended temperature, the entire content of thepreheating container can be delivered all at once, i.e. in batches, tothe heat treatment container, whereupon the preheating container can becharged with fresh polyester material. Due to the short residence time,the preheating container is operable well in a discontinuous operation.

Due to cost advantages and in favour of a simpler construction incomparison to the above-described inert gas plants, the preheatingcontainer can be operated under vacuum, preferably at between 0.1 and 10mbar, whereby the preheating container can be heated by convection heatsince the preheating container is designed with the largest possiblesurfaces which are heated. The heating of the container surfaces may inturn be effected by electric heating rods or by a heat-transfer mediumsuch as oil flowing through or around the container surfaces. This kindof heating provides the advantage that the preheating container,provided with appropriate slides or flaps, can at the same time be usedas a sluice container for the heat treatment container in order toachieve the shortest possible residence time for the polyester underatmospheric oxygen, or in order to completely eliminate the atmosphericoxygen, respectively, beginning from a temperature of 160-180° C. Aparticularly suitable embodiment of the container comprises a preferablyheated agitator in order to keep as uniform as possible, by continuousstirring, the temperature in the container and hence the thoroughheating of the polyester.

It proves to be favourable in terms of design and costs if thepreheating container is integral with the entrance area of the heattreatment container.

For the purpose of an advantageous implementation of the starting phaseof the process according to the invention, it is provided that, at theoutlet of the heat treatment container, the temperature of the polyestermaterial is measured and the material is returned to the preheatingcontainer or the inlet of the heat treatment container if thetemperature is insufficient. Should the reactor be at a standstill for alonger period of time so that the granulate contained therein will dropbelow the heat treatment temperature, it nevertheless is unnecessary tocompletely empty the heat treatment container and to extrude thedischarged material again, but rather the polyester material is recycleduntil it reaches the required temperature at the outlet, whereby in thiscase it has also received the necessary residence time.

Since due to the necessary residence times in the preheating containerand in the heat treatment container the throughput of the polyestermaterial is lower than the usual granulation capacity of a granulationunit arranged upstream, it may turn out to be beneficial if, after itsproduction, the granulate is split up to a plurality of preheatingcontainers comprising downstream heat treatment containers, or isconveyed from one preheating container into several heat treatmentcontainers, respectively, in order to reach a balanced ratio between thethroughputs of the granulation unit and of the heat treatmentcontainers.

Advantageously, the heat-treated polyester material, after having beingdischarged from the heat treatment container, can be delivered still inthe heated state to an extruder or a melt-processing device, for examplean injection-moulding machine, so that optimal use is made of the heatstored in the material.

The device according to the invention for increasing the intrinsicviscosity of a polyester material by solid-state polymerization via aheat treatment in a heat treatment container is characterized in thatthe heat treatment container is preceded by a preheating container forheating the polyester material to a heat treatment temperature of theheat treatment container or to a temperature above that, preferably toessentially the heat treatment temperature. In the preheating container,the polyester material can be brought to the intended temperature in afar shorter amount of time than would be possible solely in the heattreatment container. Advantageously, a vacuum, preferably of between 0.1and 10 mbar, can be applied to the preheating container, wherein thepreheating container can be provided with a heated agitator for fasterheating of the granulate contained therein. A discontinuous operation ofthe preheating container can be achieved if the latter is connected withthe heat treatment container via a slide.

In order to avoid the known problems associated with inert gas cycles,in a perferred embodiment of the invention, a vacuum, preferably ofbetween 0.1 and 10 mbar, can be applied to the heat treatment container.Such a plant is cost-efficient also in smaller and medium-sizedinstallation sizes.

By means of the device according to the invention it is possible tooperate the preheating container discontinuously, whereby only shortresidence times of the polyester material are necessary, however, whilerunning the heat treatment container, which must contain the polyestermaterial for the solid-state polymerization throughout a substantiallylonger residence time of 3-10 hours and more, in a continuous operation,since the material is introduced already at the intended heat treatmenttemperature. Since, in contrast to the prior art, a lower heatingcapacity of the heat treatment container is thus required (given thatthe granulate has to be kept merely at the charging temperature orslightly below that temperature without having to be heated anyfurther), the heat treatment container may be provided with wallheating, hence avoiding any problems associated with dead spaces as aresult of internal heating, or may even be an unheated, heat-insulatedcontainer.

The most simple design is achieved if the preheating container isintegral with the entrance area of the heat treatment container.Furthermore, a temperature sensor for measuring the temperature of thepolyester material and a conduit for returning the polyester material tothe preheating container or the inlet of the heat treatment containercan be provided at the outlet of the heat treatment container, with thepolyester-material discharge stream being redirectable into the returnconduit depending on its measured temperature.

In order to better adapt the device according to the invention to thethroughput of granulation units which are arranged upstream, a pluralityof preheating containers comprising downstream heat treatment containersor one preheating container comprising a plurality of downstream heattreatment containers is/are provided. In order to minimize heat losses,an extruder or a melt-processing device, for example aninjection-moulding machine, can in turn be arranged directly downstreamof the heat treatment container.

The invention will now be described in further detail by way ofnon-limiting exemplary embodiments.

In the drawings, FIGS. 1 to 4 show four embodiments of the invention inschematic diagrams, wherein equal or similar components are providedwith equal reference numerals and are illustrated only once in thesubsequent specification.

FIG. 1 shows a first embodiment of the invention comprising a preheatingcontainer 2 having a vacuum-tight inlet flap 1 into which a conduit 15for the supply of polyester material runs. The preheating container 2has a double-walled design and is heated via a heating device 3 whichheats a liquid heat-transfer medium (oil, water) that is introducedthrough heat-supply conduits 16 a into the interspace (wall heating 10)of the double walls of the preheating container 2 and is returned viadrains 16 b to the heating device 3 after having given off its heat. Inaddition, the preheating container 2 is heated from the inside by anagitator 4 heated via a rotary transmission leadthrough 4 a. Theagitator 4 is driven by a motor 17. An outlet slide 5 establishes alockable connection between the outlet of the preheating container 2 andthe actual heat treatment container 6. Via a vacuum unit 7, bothcontainers are evacuated independently by valve controls 8 and 9. Theheat treatment container 6 is only provided with a wall heating system10 a in which a heat-transfer medium circulates from the heating device3 via the supply and return conduits 16 a, 16 b. The wall heating 10 isdimensioned such that a polyester material located in the interior ofthe container will be maintained at its heat treatment temperature butwill not be heated to higher temperatures. The heat treatment container6 is surrounded by heat insulation 11 in order to compensate for theheat radiation losses. At the discharge end, there is a twin sluicesystem 12 at the heat treatment container 6 in order to ensure acontinuous vaccum in the heat treatment container while guaranteeing acontinuous discharge of material. Instead of the discharge-end twinsluice system, a vacuum-tight cellular wheel feeder might also be used.

In an experimental setup, a heat treatment container having a volume ofapprox. 1000 litres was used, the volume of the preheating containerarranged upstream amounted to approx. 40 litres.

The process according to the invention is carried out as follows in thedevice according to FIG. 1.

Via appropriate conveyor means such as a vacuum conveyor, a feed screwor conduit 15, the polyester material, f.i. PET, is conveyed via theopen inlet flap 1 into the preheating container 2, which was preheatedto a heat treatment temperature of 220° C. The outlet slide 5 of thepreheating container 2 to the heat treatment container 4 is closed. Thevalve 8 of the vacuum unit 7 to the preheating container is closed, thevalve 9 of the vacuum unit 7 to the heat treatment container 6 is openso that only the heat treatment container 6 is evacuated. After fillingthe preheating container 2 with a charge of the polyester material, theinlet flap 1 and the vacuum valve 9 to the heat treatment container 6are closed so that the generated vacuum is maintained in the heattreatment container. Thereupon, the vacuum valve 8 to the preheatingcontainer 2 is opened and the container is evacuated (preferably tobelow 5 mbar). Subsequently, the vacuum valve 9 to the heat treatmentcontainer 6 is reopened. The charge of polyester material introducedinto the preheating container 2 is heated to the heat treatmenttemperature of approx. 220° C. After the polyester material has reachedthe desired temperature, the material charge is delivered under vacuumto the heat treatment container 6 by opening the outlet slide 5 untilthe content of the preheating container has completely entered the heattreatment container 6, whereupon the outlet slide 5 is closed and thepreheating container can be filled with fresh polyester material. Afterreaching the desired residence time in the heat treatment container 6,polyester material is discharged from the heat treatment container 6through the twin sluice system 12 according to the FIFO principle (firstin first out) (in the experimental setup, the average dischargetemperature of the material from the heat treatment container amountedto 212° C.).

FIG. 2 schematically shows a somewhat simpler embodiment of the deviceillustrated above, wherein a slight oxidative degradation of thematerial (yellow colouring) in the preheating container is put up with,which may be tolerated in recycling applications. The embodiment of FIG.2 differs from that of FIG. 1 especially in that the preheatingcontainer 2 is provided neither with an agitator nor with a convectionwall heating system. Instead of by the convection heat of the containersurfaces, the polyester material in the preheating container 2 is heatedby a heat-transfer medium, in the present case air. The air is drawn inthrough a fan 18 in the form of ambient air, is heated to the desiredheat treatment temperature of f.i. 220° C. in an air heater 19, issupplied through an inlet valve 20 to the preheating container 2, ispassed through the preheating container and hence through the polyestermaterial located therein and is sucked off via an outlet valve 21.Better heating of the polyester material might be achieved by nitrogenor dry air. Nevertheless, acceptable values were achieved also with thissimple embodiment by heating the polyester material with ambient air.

A further development of the embodiment of FIG. 2 comprising air heatingof the preheating container is illustrated in FIG. 3. Thereby, thepreheating container 2 is divided into two different temperature zones,whereby, in the upper zone 2 a, a temperature of the introducedpolyester material of f.i. 180° C. is achieved via a wall heating system10 and a heated agitator 4, which temperature lies above thecrystallization temperature of the polyester material. The heating inthe upper area could also be effected by hot air. In the lower area ofthe preheating container 2, a temperature zone 2 b of approx. 220° C. iscreated by supplying hot air through the inlet valve 20, with the hotair being sucked off on the opposite side through the outlet valve 21.The pretreated polyester material is discharged via the sluice system 12and is supplied to the heat treatment container 6 via a vacuum-tightsupply conduit 22. Thus, the preheating container functions as acrystallizer in which the polyester is heated to a temperature above thecrystallization temperature in order to separate granulated polyesterbodies stuck to each other, thus restoring the flowability of the grainsfor further treatment, which is of great importance for processingprocedures taking place without agitator. Furthermore, the granulate, inits crystalline form, absorbs less moisture and also permits shorterresidence times during drying (f.i. after previous washings).

FIG. 4 finally shows a particularly simple embodiment of the inventionproducible at low cost, wherein the preheating container 2′ comprisingthe agitator 4 is integral with the upper section of the heat treatmentcontainer 6′. Furthermore, a temperature sensor 13 for measuring thetemperature of the polyester material and a conduit 14 for returning thepolyester material to the preheating container 2′ are provided at theoutlet of the heat treatment container 6′, whereby thepolyester-material discharge stream is returned to the return conduit ifthe temperature is too low. Thus, in the starting phase of the device orafter a longer standstill because of which the polyester materiallocated in the heat treatment container drops below the heat treatmenttemperature, it can be omitted to completely empty the heat treatmentcontainer and to extrude the discharged material and to reintroduce itinto the treatment process, but rather the polyester material isrecycled until it reaches the required temperature at the outlet,whereby in this case it has also received the necessary residence time.

1. A process for increasing the intrinsic viscosity of a polyestermaterial by solid-state polymerization, wherein the polyester materialis heat-treated in a heat treatment container and the polyester materialis introduced into a preheating container prior to being conveyed intothe heat treatment container, characterized in that the polyestermaterial is heated in the preheating container under vacuum, preferablyat between 0.1 and 10 mbar, to a heat treatment temperature of the heattreatment container or to a temperature above that, preferably toessentially the heat treatment temperature, with the heat transfer tothe polyester material being effected in the preheating container bymeans of a heated agitator, and the polyester material is delivered tothe heat treatment container after having reached said temperature, withthe heat treatment container being maintained under vacuum, preferablyat between 0.1 and 10 mbar.
 2. A process according to claim 1,characterized in that the preheating container has a double-walleddesign and is heated by conducting a heat-transfer medium through thecavity of the double wall.
 3. A process according to claim 1,characterized in that the heated polyester material is delivered inbatches from the preheating container to the heat treatment container.4. A process according to claim 1, characterized in that the heattreatment temperature amounts to at least 180° C.
 5. A process accordingto claim 1, characterized in that the heat treatment container is heatedvia wall heating.
 6. A process according to claim 1, characterized inthat the preheating container is integral with the entrance area of theheat treatment container.
 7. A process according to claim 1,characterized in that, at the outlet of the heat treatment container,the temperature of the polyester material is measured and the polyestermaterial is returned to the preheating container or the inlet of theheat treatment container if the temperature is insufficient.
 8. Aprocess according to claim 1, characterized in that the polyestermaterial to be treated is supplied to a plurality of preheatingcontainers comprising downstream heat treatment containers or to onepreheating container comprising several downstream heat treatmentcontainers.
 9. A process according to claim 1, characterized in that thepolyester material to be treated is granulated prior to being introducedinto the preheating container.
 10. A process according to claim 1,characterized in that the heat-treated polyester material, after havingbeing been discharged from the heat treatment container, is deliveredstill in the heated state to an extruder or a melt-processing device,for example an injection-moulding machine.
 11. A device for increasingthe intrinsic viscosity of a polyester material by solid-statepolymerization via a heat treatment in a heat treatment container,characterized in that the heat treatment container (6) is preceded by apreheating container (2) for heating the polyester material to a heattreatment temperature of the heat treatment container or to atemperature above that, preferably to essentially the heat treatmenttemperature, wherein a vacuum, preferably of between 0.1 and 10 mbar,can be applied to the preheating container (2) and to the heat treatmentcontainer (6) and the preheating container (2) is provided with a heatedagitator (4).
 12. A device according to claim 11, characterized in thatthe preheating container (2) is connected with the heat treatmentcontainer (6) via a slide (5).
 13. A device according to claim 11,characterized in that the heat treatment container (6) is provided withwall heating (10) or is an unheated container with heat insulation (11).14. A device according to claim 11, characterized in that the preheatingcontainer (2′) is integral with the entrance area of the heat treatmentcontainer (6′). (FIG. 4)
 15. A device according to claim 11,characterized in that a temperature sensor (13) for measuring thetemperature of the polyester material and a conduit (14) for returningthe polyester material to the preheating container or the inlet of theheat treatment container is provided at the outlet of the heat treatmentcontainer (6′), with the polyester-material discharge stream beingredirectable into the return conduit depending on its measuredtemperature.
 16. A device according to claim 11, characterized in thatit exhibits a plurality of preheating containers comprising downstreamheat treatment containers or one preheating container comprising aplurality of downstream heat treatment containers.
 17. A deviceaccording to claim 11, characterized in that the preheating container ispreceded by a granulation unit for the polyester material.
 18. A deviceaccording to claim 11, characterized in that an extruder or amelt-processing device, for example an injection-moulding machine, isarranged downstream of the heat treatment container.